Polarisation of all available comets’ dust versus phase angle. The black solid line and shaded areas denote the average trend of the dependence and its 3σ error, respectively. As in Fig. 6, open and filled symbols denote the pre- and post-perihelion data, while background grey circles represent data points discarded from the fitting because of either their large aperture size (≫10 000 km in cometocentric distance) and/or the possible gas contamination. All symbols are the archival data of Kiselev et al. (2017), except for the four points of 2I/Borisov (Bagnulo et al. 2021) and three points of 67P (this study). We exclude unique comets C/1995 O1 (Hale-Bopp) and 2I/Borisov from the fitting, since their P_r has been found to be significantly higher. The red diamonds edged with black lines are our spectropolarimetric data. The best fit parameters of the fitted curve are b = 25.69 ± 1.27%, c₁ = 0.67 ± 0.02, c₂ = (1.54 ± 0.85) × 10^-13, and α₀ = 21°.25 ± 0°.14. A detailed description of the legend is provided in the text.